Concise enantioselective synthesis of ent-malbrancheamide B

Total Synthesis of (+)-Brevianamides A and B

(+)-Brevianamides A (1a) and B (1b) are distinguished by their unique bicyclo[2.2.2]diazaoctane structure and have captured the interest of synthetic chemists due to their fascinating array of biological activities. The biosynthetic proposal of these classes of alkaloids led to the discovery of a number of interesting strategies. We present a biomimetic synthesis of these alkaloids starting from naturally occurring 4-hydroxy-l-proline and L-tryptophan. Gratifyingly, we emulate an alternative biosynthetic process through a unique elimination-isomerization sequence triggered by a dual-base system to generate the key aza-diene required for the Diels-Alder reaction to craft the bicyclo[2.2.2]diazaoctane structure.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Unified total synthesis of the brevianamide alkaloids enabled by chemical investigations into their biosynthesis

The bicyclo[2.2.2]diazaoctane alkaloids are a vast group of natural products which have been the focus of attention from the scientific community for several decades. This interest stems from their broad range of biological activities, their diverse biosynthetic origins, and their topologically complex structures, which combined make them enticing targets for chemical synthesis. In this article, full details of our synthetic studies into the chemical feasibility of a proposed network of biosynthetic pathways towards the brevianamide family of bicyclo[2.2.2]diazaoctane alkaloids are disclosed. Insights into issues of reactivity and selectivity in the biosynthesis of these structures have aided the development of a unified biomimetic synthetic strategy, which has resulted in the total synthesis of all known bicyclo[2.2.2]diazaoctane brevianamides and the anticipation of an as-yet-undiscovered congener.

A general approach to 2,2-disubstituted indoxyls: total synthesis of brevianamide A and trigonoliimine C

The indoxyl unit is a common structural motif in alkaloid natural products and bioactive compounds. Here, we report a general method that transforms readily available 2-substituted indoles into 2,2-disubstituted indoxyls via nucleophile coupling with a 2-alkoxyindoxyl intermediate and showcase its utility in short total syntheses of the alkaloids brevianamide A (7 steps) and trigonoliimine C (6 steps). The developed method is operationally simple and demonstrates broad scope in terms of nucleophile identity and indole substitution, tolerating 2-alkyl substituents and free indole N-H groups, elements beyond the scope of most prior approaches. Spirocyclic indoxyl products are also accessible via intramolecular nucleophilic trapping.

Solid-Phase Total Synthesis of Dehydrotryptophan-Bearing Cyclic Peptides Tunicyclin B, Sclerotide A, CDA3a, and CDA4a using a Protected β-Hydroxytryptophan Building Block

A new approach to the synthesis of Z-dehydrotryptophan (ΔTrp) peptides is described. This approach uses Fmoc-β-HOTrp(Boc)(TBS)-OH as a building block, which is readily prepared in high yield and incorporated into peptides using solid-phase Fmoc chemistry. The tert-butyldimethylsilyl-protected indolic alcohol is eliminated during global deprotection/resin cleavage to give ΔTrp peptides exclusively as the thermodynamically favored Z isomer. This approach was applied to the solid-phase synthesis of tunicyclin B, sclerotide A, CDA3a, and CDA4a.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

First total synthesis of ent-asperparaline C and assignment of the absolute configuration of asperparaline C

The first asymmetric total synthesis of a member of the asperparaline family was accomplished and the unknown absolute configuration of asperparaline C has been determined to be all-(S).

Sequential Oxidative and Reductive Radical Cyclization Approach toward Asperparaline C and Synthesis of Its 8-Oxo Analogue

The most advanced approach, so far, to the asperparalines is developed. Consecutive oxidative and reductive radical cyclizations serve as the key steps to stereoselectively access the complex fully elaborated skeleton containing the cyclopentane and spiro-succinimide units.

Oxidative radical cyclizations of diketopiperazines bearing an amidomalonate unit. Heterointermediate reaction sequences toward the asperparalines and stephacidins

A novel approach to the diazabicyclo[2.2.2]octane core of prenylated bridged diketopiperazine alkaloids is described by direct oxidative cyclizations of functionalized diketopiperazines mediated by ferrocenium hexafluorophosphate or the Mn(OAc)₃•2H₂O/Cu(OTf)₂ system. Divergent reaction pathways take place depending on the substitution pattern of the substrates and the oxidation conditions such as temperature or the presence or absence of persistent radical TEMPO. For ester-substituted diketopiperazines, the ester group exerts a significant influence on the reaction outcome and stereochemistry of the radical cyclizations.

Unified Approach to Prenylated Indole Alkaloids: Total Syntheses of (-)-17-Hydroxy-Citrinalin B, (+)-Stephacidin A, and (+)-Notoamide I

A unified strategy for the synthesis of congeners of the prenylated indole alkaloids is presented. This strategy has yielded the first synthesis of the natural product (-)-17-hydroxy-citrinalin B as well as syntheses of (+)-stephacidin A and (+)-notoamide I. An enolate addition to an in situ generated isocyanate was utilized in forging a key bicyclo[2.2.2]diazaoctane moiety, and in this way connected the two structural classes of the prenylated indole alkaloids through synthesis.

An analysis of the complementary stereoselective alkylations of imidazolidinone derivatives toward α-quaternary proline-based amino amides

Alkylations of proline-based imidazolidinones are described based on the principle of self-regeneration of stereocenters (SRS), affording high levels of either the cis or trans configured products. Stereoselectivity is dictated solely on the nature of the "temporary" group, where isobutyraldehyde-derived imidazolidinones provide the cis configured products and 1-naphthaldehyde-derived imidazolidinones afford the complementary trans configured products. These stereodivergent products can be readily cleaved to afford both α-alkylated proline enantiomers from readily available L-proline. A series of imidazolidinones were alkylated to investigate the origin of the anti-selectivity. Potential contributions toward the observed anti-selectivity are discussed on the basis of these experiments, suggesting a refined hypothesis for selectivity may be in order.

Highly enantioselective access to diketopiperazines via cinchona alkaloid catalyzed Michael additions

Alkaloid catalysed additions to triketopiperazines gives products in high yield and er (88 : 12 to 99 : 1), including bridged hydroxy-DKPs via Michael-addition–ring closure.

Michael addition reactions of triketopiperazine (TKP) derivatives to enones, mediated by a cinchona alkaloid-derived catalyst, deliver products in high yield and enantiomeric ratio (er). Use of unsaturated ester, nitrile or sulfone partners gives bridged hydroxy-diketopiperazine (DKP) products resulting from a novel Michael addition–ring closure.

Total synthesis of aspeverin via an iodine(III)-mediated oxidative cyclization

The first total synthesis of aspeverin, a prenylated indole alkaloid isolated from Aspergillus versicolor in 2013, is described. Key steps utilized to assemble the core structure of the target include a highly diastereoselective Diels-Alder reaction, a Curtius rearrangement, and a unique strategy for installation of the geminal dimethyl group. A novel iodine(III)-initiated cyclization was then used to install the bicyclic urethane linkage distinctive to the natural product.

A general protocol to afford enantioenriched linear homoprenylic amines

The reaction of a readily obtained chiral branched homoprenylamonium salt with a range of aldehydes, including aliphatic substrates, affords the corresponding linear isomers in good yields and enantioselectivities.

The cascade radical cyclisation approach to prenylated alkaloids: synthesis of stephacidin A and notoamide B

A strategy for the synthesis of members of the prenylated indole alkaloid family is described, which involves a radical cascade process of an appropriately substituted diketopiperazine (DKP) core structure. Several approaches to the generation of the initial radical were explored, with the most successful involving treatment of a sulfenyl substituted DKP under classical reductive conditions by heating with Bu3SnH and a radical initiator. The required, fully substituted, radical precursor DKP structures were prepared using regio- and stereocontrolled enolate chemistry of simpler proline-tryptophan derived DKPs. The new approach allowed rapid access to a key polycyclic indoline structure, which was converted into either of the natural products stephacidin A or notoamide B.

Direct nucleophilic addition to N-alkoxyamides

While the synthesis of amide bonds is now one of the most reliable organic reactions, functionalization of amide carbonyl groups has been a long-standing issue due to their high stability. As an ongoing program aimed at practical transformation of amides, we developed a direct nucleophilic addition to N-alkoxyamides to access multisubstituted amines. The reaction enabled installation of two different functional groups to amide carbonyl groups in one pot. The N-alkoxy group played important roles in this reaction. First, it removed the requirement for an extra preactivation step prior to nucleophilic addition to activate inert amide carbonyl groups. Second, the N-alkoxy group formed a five-membered chelated complex after the first nucleophilic addition, resulting in suppression of an extra addition of the first nucleophile. While diisobutylaluminum hydride (DIBAL-H) and organolithium reagents were suitable as the first nucleophile, allylation, cyanation, and vinylation were possible in the second addition including inter- and intramolecular reactions. The yields were generally high, even in the synthesis of sterically hindered α-trisubstituted amines. The reaction exhibited wide substrate scope, including acyclic amides, five- and six-membered lactams, and macrolactams.

Enantioselective approach to 13a-methylphenanthroindolizidine alkaloids

The first enantioselective approach to 13a-methylphenanthroindolizidine alkaloids is reported, featuring an efficient stereoselective Seebach's alkylation and Pictet-Spengler cyclization. The proposed and other three most probable structures were ruled out, indicating hypoestestatin 1 needs further assignment.

Utilizing DART mass spectrometry to pinpoint halogenated metabolites from a marine invertebrate-derived fungus

Prenylated indole alkaloids are a diverse group of fungal secondary metabolites and represent an important biosynthetic class. In this study we have identified new halogenated prenyl-indole alkaloids from an invertebrate-derived Malbranchea graminicola strain. Using direct analysis in real time (DART) mass spectrometry, these compounds were initially detected from hyphae of the fungus grown on agar plates, without the need for any organic extraction. Subsequently, the metabolites were isolated from liquid culture in artificial seawater. The structures of two novel chlorinated metabolites, named (-)-spiromalbramide and (+)-isomalbrancheamide B, provide additional insights into the assembly of the malbrancheamide compound family. Remarkably, two new brominated analogues, (+)-malbrancheamide C and (+)-isomalbrancheamide C, were produced by enriching the growth medium with bromine salts.

Synthesis of (+/-)-eusynstyelamide A

The synthesis of (+/-)-eusynstyelamide A has been accomplished in six steps in 13% overall yield from 6-bromoindole, methyl glycidate, and Boc-protected agmatine. If oxygen is carefully excluded from the reaction, the key NaOH-catalyzed aldol dimerization of the alpha-ketoamide proceeded efficiently to give Boc-protected eusynstyelamide A.

Synthetic approaches to the bicyclo[2.2.2]diazaoctane ring system common to the paraherquamides, stephacidins and related prenylated indole alkaloids

The bicyclo[2.2.2]diazaoctane ring system is common to a number of highly biologically active secondary metabolites isolated from numerous species of fungi. In this tutorial review, we describe the varied synthetic approaches that have been employed to construct this ring system in the course of recent total synthesis endeavors, and this review should be of interest to synthetic organic chemists and natural product chemists. Detailed herein are a number of synthetic disconnections including intramolecular S(N)2' cyclizations, biomimetic Diels-Alder reactions, radical cyclizations, and cationic cascade reactions.